The invention relates to a process to cool and pelletize molten strands, which issue from nozzles and are first stretched in the region following the nozzles and then are cooled for the pelletizing operation.
Devices to cool and pelletize thermoplastic strands are known in various embodiments. Thus, the DE-PS 25 03 455 shows a device with a discharge channel, over which a current of cooling water is guided into which the strands issuing from the nozzles flow. Furthermore, a device is known from the DE-OS 2 230 187, wherein the strands are guided by means of two opposing, adjacent revolving belts in which the strands and the cooling water are introduced. These devices are based on the principle of rapidly subjecting the strands, guided through a short stretch of air in which they virtually do not cool themselves, to the chilling effect of the cooling water and transporting the strands, enveloped by cooling water to the feed rollers of a pelletizer. In so doing, the strands arrive in a state in the pelletizer in which at least their surface is solidified. The devices can also be designed or operated in such a manner that a continuous solidification of the strands upon entry into the pelletizer is achieved.
Furthermore, it is known from the DE-PS 39 00 250 to divide a discharge channel of a device to cool and pelletize strands that is fed with cooling water into several zones, and in particular in such a manner that first, as in the case of the device according to the DE-PS 25 03 455, a discharge channel fed with cooling water is provided that is following by a dewatering region, in which the water flowing from the discharge channel is largely drawn off. The dewatering region is followed by a drying region, in which a current of gas is led on the strands and by means of which the residual water still adhering to the strands is almost completely removed. These three zones are in succession in the frame of the discharge channel, which brings about a distinct cooling effect only in its first region, guiding the cooling liquid.
It is also known from the DE-AS 21 61 067 to first stretch plastic strands after they have issued in the molten state from the nozzles, whereby for the purpose of preventing the strands from adhering their surface is cooled first by a flood of cooling water immediately behind the nozzles. Thereafter the strands are guided unimpeded through the air to a feed roller, which causes the strands to be stretched. Then the strands are cooled for the subsequent pelletizing. Preventing the strands from adhering with the flood of cooling water and then freely guiding the strands through the air leads to a purely superficial hardening of the strands, so that the result is a highly nonuniform degree of hardening over the cross section of the strands, a feature that is undesired for stretching.
Furthermore, it is known from the DE-OS 36 23 157 to convey molten strands issuing from the nozzles by a conveyor belt without cooling, wherein the strands are supposed to adhere to the conveyor belt, so that the conveyor belt can exert a pulling action on the strands.
The invention is based on the problem of providing a process to cool and pelletize molten strands issuing from nozzles, with which plastics can be processed that exhibit a relatively high thermal conductivity and include in particular plastics reinforced with fibers. Fiber reinforced plastics are extruded like unreinforced plastics in the molten state out of nozzles and subsequently pelletized.
The problem is solved according to the invention in that the strands are gently cooled up to a kneadable state while stretching and compacting along a tempering region, which is designed as a fluidized bed driven with gas, in particular air, and subsequently conveyed into an intensive cooling region, where the strands are chilled with cooling water and are substantially hardened, while feeding so as to cause stretching, for the subsequently pelletizing.
A fundamentally different method than the previously customary cooling process is proposed with the invention, namely while avoiding an immediate rapid cooling, a first subsequent gentle cooling, which causes the plastic to contact radially in the sense of compacting while stretching, so that the plastic material is conveyed in a state in which it hardly deforms any more when being subsequently chilled in the intensive cooling region. In so doing, a gas driven fluidized bed is used in the tempering region; on the one hand, said bed prevents the plastic strands from adhering by causing them to be conveyed without impediment and, on the other hand, it induces a gentle cooling.
The intensive cooling region is designed advantageously as a strand guide filled with cooling water, for which in particular the aforementioned discharge channel and the strand guide with two revolving belts are suitable.